The present invention relates to an optical information recording medium on and from which information is written and read by laser beam irradiation and, more particularly, to a phase-change optical disk on which information is written by changing the phase state of a recording layer and from which information is read by utilizing a change in optical characteristics between an amorphous state and crystalline state of the recording layer.
As an optical information recording medium that utilizes the recent optical technique, a magneto-optical disk or phase-change optical disk is provided. Of these disks, information is written on the phase-change optical disk by changing the recording layer between the crystalline state and amorphous state, and information is read from the phase-change optical disk by utilizing a difference in optical reflectance or optical transmittance between the crystalline state and amorphous state of the recording layer. In a conventional phase-change optical disk, generally, an absorptivity Aa in the amorphous state is higher than an absorptivity Ac in the crystalline state.
To increase the recording density, the pitches of the recording tracks of the phase-change optical disk may be narrowed. When, however, a certain recording track is irradiated with a laser beam to change the phase (crystalline state) of the recording layer, thus performing recording, temperature rise and crystallization, or so-called cross erasure, occurs due to light absorption in an amorphous recording mark with a high absorptivity, which is present on an adjacent recording track. To prevent this cross erasure, it is effective to decrease the absorptivity Aa in the amorphous state to be lower than the absorptivity Ac in the crystalline state, as described in Japanese Patent Laid-Open No. 2000-105946.
In the optical information recording described above, a technique that increases the numerical aperture (NA) of an objective lens (optical system) to be used in an optical head to as high as 0.85 has been proposed in recent years to improve the recording density. According to this technique, a recording layer is irradiated with a laser beam through a light-transmitting layer of an optical disk which is formed on the opposite side of the support substrate. The films of the respective layers comprising a reflecting layer, the recording layer, and the like are accordingly formed in an order opposite to the conventional order. However, since the reflecting layer made of a metal film or the like with a large particle size is formed immediately on the substrate, the reflecting surface has a low flatness, and the optical noise increases. Since the numerical aperture (NA) is increased to reduce the beam spot diameter, the time taken for the beam to pass through a certain small point of the recording layer decreases. Hence, the time during which the recording layer is held at a crystallization temperature or higher decreases, and an erasure ratio decreases.
As described above, the recording density cannot be easily improved by merely increasing the numerical aperture (NA) of the optical system of the optical head.
It is an object of the present invention to provide a phase-change optical disk in which the recording density is improved by increasing the NA of the optical system of an optical head.
In order to achieve the above object, according to the present invention, there is provided a phase-change optical disk which comprises a first dielectric layer, reflecting layer, first interface layer, recording layer, second interface layer, second dielectric layer, and light-transmitting layer sequentially formed on a support substrate, and writes thereon, reads therefrom, and erases therefrom information by changing a crystalline state of the recording layer with laser beam irradiation from a light-transmitting layer side, wherein a disk reflectance Rc obtained when the recording layer is in a crystalline state is not less than 3% and not more than 12%, a difference in absolute value between the disk reflectance Rc and a disk reflectance Ra which is obtained when the recording layer is in an amorphous state is not less than 10%, a refractive index of one of the first and second interface layers for the laser beam with a wavelength xcex is higher than 2, and the reflecting layer contains, as a main component, either one of one element selected from the group consisting of silver, cupper, titanium, and nickel and a mixture of at least two elements selected from the group consisting of silver, cupper, titanium, and nickel.